1. Field of the Invention
The present invention relates to a method of filtering noise in capacitive touch panel. More specifically, the present invention relates to a filtering noise method in capacitive touch panel by comparing the results of correlation scans.
2. Description of Related Art
With the development of capacitive touch panel, more and more users get used to capacitive touch panel. But the expensive price of capacitive touch panel systems hindered the market. Capacitive touch panel is sensitive to electrical source noise, RF noise and LCD noise. The electrical source noise could be classified into power frequency noise (50/60 Hz) and power switch noise. The power frequency noise is strong in magnitude, generally ranged from tens to two hundred volts. If the circuit design is not proper, the system might be failed due to power frequency noise. Different power switch will produce different power switch noise. The frequency of power switch noise, which ranged from tens to hundreds KHz, is very close to the signal scan frequency of touch panel system. Thus, the signal will be easily disturbed.
RF noise is another noise source of touch panel system. Generally, a frequency of more than 1 GHz will not disturb the touch panel system because such high frequency signal will be immediately attenuated by circuits. But the envelope signal of these high frequency signals, which has a lower frequency, is likely to disturb the touch panel system.
LCD noise becomes more and more important concern. Modern design tries to eliminate the thickness by reducing the distance between the touch panel and LCD or by eliminating the thickness of the touch panel itself. The frequency of LCD line drive signal is about tens of kHz, which will disturb normal scan frequency. The capacitive touch panel noise has the following characteristics: (1) Definiteness: Some of the noise is definite once the system is built. For example, the power source noise including working frequency noise and power switch noise is closely related to the power switch and is determined by power switch. The RF noise between terminal and touch panel is definite, such as GSM, CDMA, WiFi, Bluetooth and etc. LCD noise is also definite. Thus, the magnitude of touch panel system's noise and frequency are basically definite. Further, the systems using touch panels are often portable devices, they are subject to changing outside environment. The parameters of the environment such as power source and RF noise are basically definite. In a word, touch panel systems are affected by definite noise under normal use. Only under exceptional cases will the touch panel systems be affected by indefinite noise. When the systems are affected by indefinite noise, short-term failure (ex: under 100 mS) is acceptable. (2) Duration Is Short: The frequency of power switch noise is not high (tens of kHz), and the duration is very short, only several micro seconds. During the cycle of noise signal, the noise only lasts for a very short time. For the LCD screen, the scan frequency is tens of Hz. Because there are hundreds of lines, the disturbance duration of each line only represents one hundredth of a cycle. In the application U.S. 20080162997A1, three different frequencies are used when one scans noise, and the results of the scans will be compared to determine one reliable result. With the developments of more and more applications of the capacitive touch panel, cost down becomes a very important issue. The prior art is not cost-effective because it requires all noises should be limited in a small frequency zone, and it is expensive to meet such requirement.
One should carefully consider the RF disturbance during the design stage of the capacitive touch panel. The structure design of the terminal is complex and many factors contribute to it. In order to reduce the RF disturbance, certain compromises will be made: it maybe thicker, the reliability maybe lower, or more cost should be spent. Adding a mask to the touch panel also increase the cost and complexity of the structure.
Currently, LCD noise is prevented by adding a mask layer or mask electrode. Accompanying the mask layer, it often requires an ITO glass layer or ITO film layer, which increases both cost and thickness. There are many shortcomings in the method mentioned in U.S. 20080162997A1 application: First, the relation among many frequencies is not close, which adds difficulty to the comparison process. Second, if the used frequency is already disturbed, then the result becomes meaningless.